Process of operating a blast furnace by varying gaseous feed rates

ABSTRACT

A METHOD OF OPERATING A BLAST-FURNACE COMPRISING BLOWING INTO THE FURNACE A GASEOUS AUXILIARY REDUCING MEDIUM, SUCH AS REDUCING GAS, AT A LEVEL ABOVE THE BLAST TUYERES AND SUBSTANTIALLY ABOVE THE ZONE IN WHICH THE BURDEN MELTS DOWN, WHILE SIMULTANEOUSLY BLOWING IN BLAST AIR THROUGH THE BLAST TUYERES, WHEREIN THE FEED RATE OF SAID REDUCTION MEDIUM AND THE FEED RATE OF SAID BLAST AIR ARE BOTH PERIODICALLY VARIED, EACH BETWEEN A MAXIMUM AND A MINIMUM VALUE, SAID FEED RATES BEING MATCHED, ONE TO THE OTHER, TO PROVIDE A SUBSTANTIALLY CONSTANT RATE OF PRODUCTION OF TOP GASES AND TO GIVE AN OPTIMUM GAS VELOCITY AT THE TOP OF THE FURNACE.

Oct. 9, 1973 w. WENZEL 3,764,299

' PROCESS OF OPERATING A BLAST FURNACE BY VARYING GASEOUS FEED RATES Filed May 26, 1971 FIG. I

J 29 T T-28 l6 l4 val l l9 I8 25 2o 6 22 United States Patent Ofice 3,764,299 Patented Oct. 9, 1973 3,764,299 PROCESS OF OPERATING A BLAST FURNACE BY VARYING GASEOUS FEED RATES Werner Wenzel, Aachen, Germany, assignor to Nippon Kokan Kahushiki Kaisha Filed May 26, 1971, Ser. No. 146,914 Claims priority, application Germany, June 20, 1970, P 20 30 468.1 Int. Cl. C211) 5/06 U.S. C]. 75-42 12 Claims ABSTRACT OF THE DISQLOSURE A method of operating a blast-furnace comprising blowing into the furnace a gaseous auxiliary reduction medium, such as reducing gas, at a level above the blast tuyeres and substantially above the zone in which the burden melts down, While simultaneously blowing in blast air through the blast tuyeres, wherein the feed rate of said reduction medium and the feed rate of said blast air are both periodically varied, each between a maximum and a minimum value, said feed rates being matched, one to the other, to provide a substantially constant rate of production of top gases and to give an optimum gas velocity at the top of the furnace.

FIELD OF THE INVENTION This invention relates to blast-furnaces and their operation.

BACKGROUND OF THE INVENTION Methods of operating blast-furnaces are known in which a finished reducing gas, which consists mainly of carbon monoxide or hydrogen or mixtures of these two components, is blown into the stack of the blast-furnace above the level of the tuyeres. The effect of this step, particularly on the reduction of the coke consumption, is limited by the fact that it is normally not possible to introduce such gases deep into the blast-furnace stack from the side. Substantial penetration of these gases towards the middle zone of the blast-furnace is inhibited by the gases that flow upwards through the stack from the well zone.

SUMMARY OF THE INVENTION The present invention consists in providing a method which enables the reducing gases that are to be blown in to be introduced considerably more deeply into the blast-furnace, it also being possible for such gases to reach the middle zone. The method of the invention consists in periodically varying, between a maximum and a minimum, the quantities of gas which are to be blown simultaneously into the blast furnace and Which consists of the combustion blast to be introduced through the blast-air tuyeres, and the auxiliary reducing gas to be blown in at a higher level through separate elements. It should be understood that by this method, alternately, on the one hand, a greater quantity of blast air and a smaller quantity of auxiliary gas, and, on the other hand, a greater quantity of auxiliary gas and a smaller quantity of blast air, are blown in. The variation in the quantities of gas can be so great that, for example, the quantity of blast air is virtually reduced to zero, While the quantity of auxiliary gas blown in is maintained at a maximum, and that, conversely, the quantity of auxiliary gas is virtually reduced to zero at the time when the quantity of blast air is at a maximum. An important feature of this invention is that the rate of production of to gas is kept at high level by means of suitable interrelated elements for controlling the two streams of gas fed to the blastfurnace so that this rate is practically constant and represents the maximum quantity of gas that can flow from the furnace top without disadvantageous effects due to excessive gas velocity.

In one embodiment of the invention, the feed rate of blast air is periodically reduced virtually to zero. During this phase the entire volume of the stack is traversed only by the auxiliary reducing gas, and there is uniform gas flow from the middle to the top of the furnace. Conversely, the feed rate of reducing gas can be periodically reduced virtually to zero, the blast air feed rate during this phase then being at its maximum. During this phase the blast-furnace is traversed by the well gas while maintaining a uniform manner flow in the upper zone of the furnace. Thus, there is maintained uniform conditions in the blast furnace, upon "which conditions proper operation of a furnace is largely dependent.

The optimum conditions for blowing in two gaseous media, and particularly the blast air, may render it necessary not to reduce the quantity of gas periodically to zero, but to a minimum value which can be for example 20% of the maximum input. This is particularly necessary as regards blowing the air through the blast air tuyeres of the blast-furnace, since otherwise there might arise the danger of molten constituents from the interior of the furnace, particularly molten slag, entering the blast tuyeres and stopping them up.

A further important feature of the invention consists in maintaining, in the furnace stack, specific temperatures at which optimum effects as regards the performance of the blast-furnace can be obtained. Thus, the position at which the auxiliary reducing gas is blown into the stack is so selected that this gas is introduced into the blastfurnace just above the zone at which melting of the ore or of the spongy iron resulting from the ore takes place. The gases rising through the smelting zone from the well of the blast-furnace have a temperature of roughly 1200 to 1300 C. at this point. The blown-in auxiliary gases have the effect of reducing this temperature as rapidly as possible to a value of about 1000 C. as a result of mixing with the auxiliary gases. This step offers the important advantage of restricting to the maximum extent, by direct reduction, the coke-consumption zone which attains a temperature of approximately 1000 C., so that a saving in coke is effected. In the alternating blow-in method of operation, this rapid cooling of the stack gases is also achieved by blowing in an auxiliary gas of appropriately low temperature, 800 C. for example. The temperature of the stack contents located above the fusion zone is periodically raised by the ascending well gases during the period in which the blast air is being injected at maximum rate, and thereafter are cooled again to the required lower temperature limit during the period in which the reducing gases are blown in.

By means of these methods of the invention, the in direct reduction as it is called, which generally involves high consumption of reducing gas, can be raised to and more, whereas the direct reduction, in which reduc tion coke is consumed, can be limited to 10% and less. The figures for coke consumption are correspondingly low when using the method of the invention.

In this mode of operation, combustion of the coke in front of the blast tuyeres of the furnace is due mainly only to the heat supplied for melting down the reduction products and for making good the losses in the walls in the lower furnace. To enable these conditions, with which very low figures for coke consumption are associated, to be established, the durations of the various blow-in periods must be so related to each other that during the period in which blow-in of auxiliary reducing gas predominates, the iron ore is largely reduced to metallic iron (sponge iron), until, on moving down the furnace stack, it reaches the zone in which the auxiliary gas and the well gas are mixed, and so that this sponge iron is largely converted into the molten state during the period in which injection of blast air predominates. Since maximum flow of well gas upward through the fusion zone occurs during this period in which the blast air is injected, the damming action of the gas on the molten products more or less inhibits the downward flow of the latter. It sufiices however if thorough fusion of the smeltable products occurs during this blow-in period. During the subsequent period in which the auxiliary reducing gas is blown in, the intensity of the gas flow through the melting-down zone is reduced to such as extent as a result of the reduction in the amount of injected blast air, that during this period the smelting products can flow unhindered downwards into the well. The optimum performance of this alternating fusion and flow-off process requires that the media blown in and the injection times be properly matched to each other. According to the invention, provision is made for just sufiicient fully reduced sponge iron to be formed during the period that the auxiliary reduction gas is blown in as can be melted down during the next period by the well gas that ascends during the subsequent period in which blast air is blown in.

Performance of the present invention necessarily involves the feature that the composition of the top gas, which remains substantially the same as regards quantity, undergoes considerable variations. If, for example, use is made of an auxiliary reducing gas consisting largely of carbon monoxide and hydrogen, then during this portion of the blow-in time, there is obtained a top gas of high calorific value which apart from containing residual carbon monoxide and hydrogen, also contains carbon dioxide, water vapour and nitrogen, the quantities of which are small in view of the small amounts of air blown in during this portion of the time. On the other hand, during the portion of the time in which the blast air is injected, a top gas is produced which corresponds substantially to a normal blast-furnace top gas with air operation and with a correspondingly high nitrogen content. According to the invention, the top gas is the various portions of the blow-in period is put to different uses depending upon the particular quality of the gas. For example, in the case where regenerated top gas is used as an auxiliary reducing gas, the gas to be regenerated can be drawn off during the portion of the period in which auxiliary reducing gas is blown in. In this case, the top gas can be passed to a water scrubber and/or carbon dioxide scrubber and/or it can be chemically regenerated by chemical reaction with fossil fuels such as oil or methane, and blown into the blast furnace stack again after heating to about 800 C. On the other hand, according to the invention, the top gas for heating the regenerators, in which the air blast for the furnace is raised to temperatures of 1100 C. for example, can be drawn off during that portion of the time during which blast air is injected. The quantities and qualities of gas used for the various purposes can be stored in separate gas storage means until they are ready for further use. A step in the method of the invention that is of importance from the point of view of economics consists in the joint operation of two or more adjacent blast-furnaces operating in accordance with the same process. This joint operation can consist in using low-grade top gases from one of the blast-furnaces for heating the air blast for the other blast-furnaces, if possible without storing the gas; on the other hand, highgrade top gases from such a blast-furnace can be used after regeneration as an auxiliary reducing gas in one or more neighbouring blast-furnaces.

In a further form of the method of the invention, oxygen-enriched air or concentrated oxygen, optionally mixed with combustion-retarding media, are used with water vapour and/or carbon dioxide as the blast air for the furnace.

A particularly simple embodiment of the invention consists in using only one regenerator in each case for preheating the combustion air blast and/or for heating the auxiliary reduction gas, and in matching the heating-up and heat delivery period of the regenerator to the period required for blowing the heated medium in question into the blast furnace.

The method of the invention can be adapted for using the reducing gas of the particular blast-furnace itself, as well as for using reducing gas from outside sources. In the first case, part of the top gas is blown back into the stack of the blast-furnace as auxiliary reducing gas after regeneration. In the second case, the reducing gas is drawn from other sources. Such sources can be gasproduction processes of known kinds, particularly those in which natural gas and/ or petroleum are reacted with oxidizing gases such as oxygen, water vapour, carbon dioxide or mixtures thereof, and optionally also with inert components. In this connection, the invention also envisages the use of such gases that are obtained from the use of various forms of nuclear energy. According to the invention, such gas-production methods are adapted to suit the blast-furnace process by matching the working periods during which the gas is produced on a timedependent basis to the blow-in periods of the blastfurnace, taking into account the gas-quantity/time function of said periods. Thus, for example, methane can be reacted with water vapour in a regenerator, gas production being adjusted to the requirements of the blastfurnace.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will now be described in more detail with reference to the accompanying drawings, in which:

FIG. 1 shows schematically a blast-furnace and associated equipment; and

FIG. 2 shows a tuyere.

DESCRIPTION OF PREFERRED EMBODIMENT A blast-furnace is shown schematically at 1; the burden 2, consisting of ore, coke and fluxing materials is introduced at the top of the furnace, i.e. at the stack. The molten products 3, that is, pig iron and slag, are drawn off from the well of the blast-furnace. The hot blast air 4 is injected at the level of the blast tuyeres of the furnace. The streams of top gas 5:: and 5b escape through the top of the furnace. Above the level of the blast tuyeres of the furnace is located the melting-down zone 6 in which the primary iron and primary slag are melted down by means of the hot well gases rising from the level of the blast tuyeres, the primary iron and primary slag then flowing into the lower furnace.

Also shown in FIG. 1 is the blast-air heater 7 which is normally a regenerative heater. This blast-air heater is heated by a part 8 of the stream of top gas 5b. The combustion air required for burning the top gas is not shown separately in the sketch. The burned gas 9 occurring upon combustion of the part-stream 8 of top gas is discharged from the blast-air heater 7. The cold blastair 10 is passed into the heater 7 and is heated to a tem perature of 1150" C. for example. Hot blast air 4 is blown into the blast-furnace at this temperature.

The arrangement shown in FIG. 1 also includes a gas scrubbing installation 11 of known construction, in which, by known methods, carbon dioxide and/or water vapour are washed out of a part-stream 12 of the top gas 5a and are exhausted into a duct 13. If, in the gas scrubbing installation 11, a process is carried out that requires a heating operation, the heating gas 14 necessary for this is drawn as a part-stream from the top gas 5b and is combusted with air which is not shown separately in this sketch. The resultant burned gas is exhausted from the scrubbing installation 11 through the duct 15. Reducing gas 16 from an outside source is optionally jointly introduced into the top gas scrubbing installation 11 with the part-stream 12 of the top-gas 5b that is to be scrubbed, said reducing gas 16 being produced in known manner from f ssil fuels and separately from the blast-fur ace plant here illustrated, and the reducing gas also contains fairly large quantities of water vapour and/or carbon dioxide.

The gas 17, that is produced in the top-gas scrubbing installation and is largely free from carbon dioxide and water vapour and mainly consists of the reducing gases carbon monoxide and hydrogen, is then passed to the gas heating installation 18. The reduction gas is heated in this installation 18 preferably on a recuperative basis. The heating gas used is constituted by a part-stream 19 of the top gas b which is combusted with air not shown separately in this sketch. The burned gas occurring during combustion is exhausted from the gas-heating installation 18 through the duct 20. Optionally, reducing gas 21 from an outside source, which is produced in known manner from fossil fuels separately from the blast-furnace plant shown in FIG. 1 and which is already largely free from carbon dioxide and water vapour, is heated together with the scrubbed reduction gas 17. The heating of the gas in the installation 18 is carried out at a gas temperature such that the reducing gas 22 can be blown in at the particular temperature required in the stack of the blastfurnace. This temperature ranges roughly between 800 and 1000 C. The gas stream 22 is introduced just above the melting-down zone 6 of the blast-furnace, the temperature of the blown-in gas being matched to the temperature of the stack gas rising from the lower furnace in such manner that the temperature of these mixed gases is approximately 1000 C.

FIG. 1 also shows the control elements 23 to 27 in the various gas ducts. The purpose of the regulating element 23 is to take off from the low-nitrogen top gas stream 5a, the stream of reducing gas that is required by the blast-furnace 1 and is passed through the gas scrubbing installation 11 and the gas heater 18 and is blown into the stack of the blast-furnace as hot reducing gas 22. The purpose of the regulator 27 is to adjust the quantity of cold blast air to the required level for blowing hot blast air 4 into the tuyeres of the blast-furnace 1. The regulating elements 24, 25 and 26 adjust the flow of hot gas from the hot-gas collecting duct 29 to the quantity required by the gas scrubbing installation 11, the gas heater 18 and the blast-air heater 7. A particular feature of the invention is constituted by the fact that the gas regulator 23 periodically releases a maximum quantity of reducing gas, whilst at the same time the regulator 27 passes a minimum quantity of blast air to the blast-air heater 7. After a certain blow-in period in this condition, the system is reversed, and the regulator 23 brings the quantity of reducing gas down to a minimum, whereas at the same time the regulator 27 raises the quantity of blast air to a maximum.

In a further form of the invention, special measures are employed for blowing in the blast air. Since the quantity of blast air is periodically reduced from a maximum quantity to a minimum quantity which can be 20% or less of the maximum quantity, special means are used to ensure that the depth of penetration of the blast air into the charge of the blast-furnace remains sufficiently great. The required depth of penetration is ensured by maintaining the velocity of the blast air at substantially the same level when it enters the furnace chamber, both when the maximum and minimum quantities of blast air are blown in. For this purpose, it is necessary to suit the injection cross-section to the quantities of blast air. According to the invention, this can be done by the use' of two different blast tuyeres having different cross-sections, one for blowing in the maximum quantities and the other for blowing in the minimum quantities of blast air, and, of these, that tuyere not operating is cut off from the hot blast-air supply duct. It is also possible to operate with only one blast tuyere, this then having two different mouths extending into the furnace chamber and with appropriately differing blow-in cross-sections. Furthermore, according to the invention, it is possible to operate with 6 one blast tuyere and only one opening directed into the furnace chamber, but in this case, the blow-in cross-section should be capable of being partially covered for the purpose of injecting the minimum quantities of blast air.

An advantageous arrangement for this latter purpose is illustrated in FIG. 2. This schematic illustration shows a blast-furnace tuyere 30 through which water flows and which has a blow-in cross-section 31. The blast-air supply pipe 32 runs to the tuyere 30. Lateral pockets 33 and 33a are fitted to this supply pipe 32. Contained in the pockets are slides 34 and 34a which are preferably made of ceramic material and can be moved into or out of the blast-air supply pipe 32 by means of rods 35 and 35a. In the upper portion of the drawing, is illustrated the position of the blast-air injection device with the slide 34 retracted, while in the lower part of the slide 34a is shown in the pushed-in position. In the middle of the blast-air supply pipe 32 is the pipe 36 for supplying auxiliary blast air, this pipe being supported or centered on stands 39. In the pushed-in position, the slides 34 and 34a each lie closely against the surface of the pipe 36 at the blast inlet end 37. In the retracted position of the slides 34 and 34a, the stream 40 of blast air can pass freely over the entire cross-section of the supply pipe 32, including the middle pipe 36. When the slides 34 and 34a are pushed in, they close off the annular chamber round the middle pipe 36, so that the stream 40 of blast air can only be supplied to the mouth 31 of the blast tuyere through the middle pipe 36. The ratio of the cross-section of pipe 32 to that of pipe 36 is such that optimum velocities in the stream of air flowing from the blast tuyere are obtained both for the case where the quantity of blast air is at its maximum and for that when this quantity is at its minimum.

What is claimed is:

1. A method of operating a blast-furnace containing a plurality of peripherally spaced blast tuyeres, and a plurality of peripherally spaced auxiliary tuyeres positioned at a level above the blast tuyeres and substantially above the zone in which the burden melts down comprising blowing an auxiliary reducing gas through said auxiliary tuyeres while simultaneously blowing blast air through the blast tuyeres, and periodically varying the feed rate of said auxiliary reducing gas through each of said auxiliary tuyeres between maximum and minimum values, and periodically vary the feed rate of said blast air between maximum and minimum values, said feed rates being (i) varied in oppo site directions so that when the feed rate of blast gas to blast tuyeres is at a maximum the feed rate of auxiliary reducing gas to auxiliary tuyeres is at a minimum, and vice versa, and (ii) matched with respect to each other to provide a substantially constant rate of production of top gas and to give an optimum gas velocity at the top of the furnace.

2. A method as specified in claim 1 in which top gas which is to be used for heating blast air is drawn off from the top gas stream during the period in which the blast air is blown into the furnace at maximum rate.

3. A method as specified in claim 1 wherein said minimum value is at least 20% of said maximum value of each of said feed rates.

4. A method as specified in claim 1 wherein the crosssection of the blast tuyere is varied reciprocally to the amount of blast-air blown through said tuyere whereby the velocity of the blast-air exiting from said tuyere is maintained constant.

5. A method as specified in claim 1 wherein the feed rates and temperatures of the auxiliary reducing gas and blast air fed to the furnace are matched to maintain a temperature, after mixing the two gas streams in the furnace, of approximately 1000 C.

6. A method as specified in claim 1 in which the top gas stream during the period in which auxiliary reducing gas is blown into the furnace at maximum rate is heated and recycled as said auxiliary reducing gas.

7. A method as specified in claim 1 in which regenerators are used for heating the blast air and the auxiliary reducing gas, and only one generator being used for each purpose.

8. A method as specified in claim 1 in which iron ore is reduced to molten iron, the duration of the periods of time during which, respectively, the auxiliary reducing gas and the blast air are blown into the furnace at their maximum rate are so matched that while the auxiliary reducing gas is being blown in at maximum rate the iron-ore is largely reduced to metallic iron in the form of sponge iron, above the level of the zone in which the auxiliary reducing gas is mixed with the well gas, and while the blast air is being blown in at its maximum rate the sponge iron, formed in the preceding period, is converted into the molten state.

9. A method as specified in claim 8 in which the duration of the periods of time during which, respectively, the auxiliary reducing gas and the blast air are blown into the furnace at their maximum rate are so matched that while the blast air is blown in at maximum rate the material in the process of being melted is held up by the upward stream, and While the auxiliary reducing gas is blown in at maximum rate the melted material flows down as a result of the corresponding reduction in the flow rate of the blast air.

10. A method as specified in claim 1 in which said auxil- 8 iary reducing gas is produced by reacting at least one fuel from the group consisting of natural gas and petroleum with an oxygen containing medium outside of the blast furnace.

11. A method as specified in claim 10 wherein said oxidising gas is at least one gas selected from the group consisting of oxygen, water vapour, and carbon dioxide and wherein the reaction takes place in a regenerator.

12. A method as specified in claim 11 wherein said oxidizing gas also contains an inert gas.

References Cited UNITED STATES PATENTS 3,188,070 6/1965 Lee 266-41 2,790,711 4/1967 Sellers et al -42 X 1,491,131 4/1924 Coffin 75-42 2,952,533 9/1960 Cuscoleca et al 75-41 2,395,385 2/ 1946 Green et al 7542 X 2,727,816 12/1955 Raick 7542 FOREIGN PATENTS 275,601 10/1928 Great Britain 7541 883,998 12/1961 Great Britain 75--42 L. DEWAYNE RUTLEDGE, Primary Examiner M. J. ANDREWS, Assistant Examiner 

